With the development of technology, the fields of application for resins have been increasingly expanded in recent years, such as parts for vehicles, parts for household electrical appliances, and various industrial materials. Secondary processing technology concerning resin joining is also one of the technologies contributing to the expansion.
As a method of joining the resin, for example, mechanical joining by screws, bolts, or the like; joining by an adhesive such as hot melt adhesives; welding represented by hot plate welding by applying heat for melting resins; vibration welding utilizing frictional heat generated by vibrating a joint portion; laser welding by irradiating a laser beam to a joint portion and utilizing absorption and heat generation at the portion, and the like can be mentioned. Recently, the usefulness of hot plate welding, vibration welding and laser welding has increased from the viewpoints of reduction of processing steps, weight reduction, reduction of environmental burden, and the like.
In hot plate welding, for example, a heated hot plate is pressed against the surfaces to be joined (bonding portions) of each of two resin members for several seconds, and after both of them are melted, they are promptly separated from the hot plate and joined. Hot plate welding is widely used. However, in hot plate welding, a threading phenomenon may occur in which the molten resin is elongated like a thread when the hot plate is separated from the bonding portion of the resin member. When this threading phenomenon becomes conspicuous, the obtained resin joined body will have a poor appearance.
In vibration welding, for example, vibration due to pressure and reciprocating motion is applied to the bonding portions of each of the two resin members, and the resin components of the bonding portions are melted by the frictional heat, thereby joining the two. Vibration welding is an excellent method capable of joining the resin members with each other in an extremely short period of time without heating the resin members. However, in this vibration welding method, a thread burr phenomenon may occur in which the molten resin protrudes like a thread on the outside of the joint portion. When this thread burr phenomenon becomes conspicuous, the obtained resin joined body will have a poor appearance.
In laser welding, usually two materials, that is, a “transmitting material” that transmits laser light and an “absorbing material” that absorbs laser light are joined. For example, when the absorbing material and the transmitting material are placed by bringing their respective bonding portions into contact and laser light is irradiated to the material contact interface from the transmitting material side in a non-contact manner, the irradiated laser light directly passes through the transmitting material, reaches the surface of the absorbing material, and is absorbed. The light energy absorbed by the surface of the absorbing material is converted into heat and melts the bonding portion. In addition, the heat of fusion is thermally transferred to the transmitting material and melts the bonding portion of the transmitting material. Thereafter, the melted bonding portions of each of the absorbing material and the transmitting material are solidified and welded with cooling. The resin joined body obtained through such a process is excellent in strength, airtightness, and appearance (no burr generation, or the like). In addition, laser welding also has features such as the following: good working environment; and when some components are incorporated in the resin member to be joined, there is little damage to the incorporated components. However, in this method, if the laser light to be irradiated is too strong, the amount of heat generated by the resin will increase, thereby causing poor appearance such as foaming, scorching, discoloration and the like. On the other hand, if the laser light to be irradiated is too weak, the joining strength decreases and, in some cases, problems such as insufficient welding may occur. Therefore, when performing laser welding, it is very important to control the amount of heat generated by the resin within an appropriate range.
Incidentally, a molded article obtained from a thermoplastic resin is often used in a state colored with a pigment (colored molded article). From the viewpoint of product design properties, the color tone of the colored molded article is regarded as very important. Although the importance of color tone is also high in parts for vehicles and the like, since the design is preferred in the market, the demand for black color in particular tends to increase every year.
Since carbon black which is a representative of a black colorant has high absorptivity of laser light, it is efficient in increasing the amount of heat generation and melting the resin.
However, when the added amount of carbon black is excessively increased, since the amount of heat generation becomes excessive, it is likely to cause poor appearance such as foaming, scorching, discoloration and the like at the time of irradiation with laser light as described above. In other words, if the added amount of carbon black is increased in order to obtain a deep black tint, the appearance after laser welding deteriorates conversely. Therefore, it is very difficult to achieve both the color tone (especially the black tint) and the appearance after laser welding.
Patent Document 1 discloses a colored styrene-based resin molded body excellent in laser marking properties and molded article appearance (surface gloss), which contains a styrene-based resin and a colorant, and the number of the colorant present in the molded body as secondary particles of 0.1 to 10 μm is from 100 to 20,000 particles/m2.
However, in this document, there is no description concerning compatibility between the color tone (particularly the black tint) of a molded article and the appearance after laser welding. In the colored styrene-based resin molded body disclosed in Patent Document 1, there is a technical problem as a laser welding material from the viewpoints of the balance between the black tint and laser light absorption and the impact resistance.
Patent Document 2 discloses a thermoplastic resin composition for a lamp housing obtained by combining, at a specific ratio, a rubber reinforced resin composed of a rubber-reinforced copolymer resin (obtained by polymerizing a vinyl-based monomer containing an aromatic vinyl compound and a vinyl cyanide compound in the presence of a rubbery polymer having a gel content of 70% or more) or a composition of the aforementioned rubber-reinforced copolymer resin and a (co)polymer of the vinyl-based monomer, and a copolymer containing a maleimide-based monomer unit and/or a (co)polymer of α-methylstyrene, in order to improve defective phenomena occurring during vibration welding, hot plate welding or laser welding in joining a lamp housing and another member.
However, in this document, there is no description concerning compatibility between the color tone (particularly the black tint) of a molded article and the appearance after laser welding. The composition disclosed in Patent Document 2 cannot sufficiently satisfy the level of demand for the black tint.